1. Field of the Invention
The present invention relates to an information reproducing apparatus that reproduces information from an information recording medium such as an optical disk, a magneto-optical disk, a compact disc (CD), or a CD-R.
2. Related Background Art
A partial Response Maximum likelihood (PRML) method is known as a signal processing method for optical information reproducing apparatuses including optical disk drives. In the PRML method, a reproduced signal is equalized by a PR method according to characteristics of a recording/reproduction system, and decoding processing for maximum likelihood decoding such as Viterbi decoding is performed to obtain data at a reduced error rate even if the reproduced signal has large intersymbol interference. Japanese Patent Application Laid-Open No. 2003-141823 discloses a method of evaluating signal quality in a reproduction system using PRML.
FIG. 10 shows the outline of this evaluation method. FIG. 10 is a trellis diagram showing state transition in PRML using PR (1, 2, 2, 1) as a PR characteristic. In the conventional method, decoded data corresponding to a predetermined path A or B in a path through which a transition from a state S0 at a time k−4 to state S5 at a time k in FIG. 10 is made is detected and metrics Ma and Mb of the paths A and B are generated from PR output values generated on the basis of a reproduced signal. Evaluation of signal quality is then performed by using the metric difference |Ma−Mb| between the generated metrics Ma and Mb. As the predetermined paths from which the above-described metric difference is detected, paths of the minimum Euclidean distance in PR (1, 2, 2, 1) for example are set. The paths shown in FIG. 10 are an example of paths of the minimum Euclidean distance. FIG. 11 shows reference amplitudes in PR (1, 2, 2, 1) with respect to the paths A and B in FIG. 10. As shown in FIG. 11, the values of reference amplitude in PR (1, 2, 2, 1) are seven values of 0 to 6. Therefore, the reference amplitude values corresponding to the path A are [0 1 3 5], and the reference amplitude values corresponding to the path B are [1 3 5 6].
Here, if the amplitude values of a reproduced signal by PR (1, 2, 2, 1) are [0.2 1.3 3.5 4.8], that is, when amplitude values indicated by P in FIG. 11 are obtained, the metrics of the paths A and B are as shown below.Ma=(0.0−0.2)2+(1.0−1.3)2+(3.0−3.5)2+(5.0−4.8)2=0.42Mb=(1.0−0.2)2+(3.0−1.3)2+(5.0−3.5)2+(6.0−4.8)2=7.22From these Ma and Mb, the metric difference |Ma−Mb|=6.8.
In the PRML decoding process, when paths of the minimum Euclidean distance are detected and only the metric differences between the paths are statistically processed, a distribution such as shown in FIG. 12 is obtained. At this time, since the minimum Euclidean distance in PRML of PR (1, 2, 2, 1) is 10, the central value of the distribution is 10. In the above-described conventional method, statistical processing of the above-described metric difference is performed and a signal evaluation index is generated from a mean value and a standard deviation or the like thereof to enable evaluation of the signal quality.
Further, the servo control value and the equalization characteristics of a waveform equalizer or the like are adjusted by using the quality evaluation index to optimize reproduction quality.
However, although the above publication describes evaluation of signal quality from the PRML metric difference and an apparatus for adjusting a servo control value and a recording power control value using the above described evaluation index with respect to the above-described conventional method, no description has been made of optimization of any Viterbi decoder for generating a decoded signal.
A reproduced signal has a distortion such as asymmetry, and a distortion component that cannot be removed by optimization of a servo system or optimization of an equalizer only cannot be prevented from being mixed in an input signal to a Viterbi decoder. This distortion component causes an error in metric to hinder decoding processing from being correctly performed.